A variety of carbon nano-materials, typically, carbon nanotubes have thus far been developed, and are considered to be promising materials with various functions in a variety of usages such as conductive fillers, thermal conduction materials, light-emitting elements, electrode materials for batteries or capacitors, wiring materials, electrode-bonding materials between wires, reinforcing materials, and black pigments.
However, in general, carbon nano-materials form an aggregate immediately after being manufactured, and it is extremely difficult to produce a state in which carbon nano-materials are sufficiently dispersed in a solvent. As a result, there is a problem in that, when made into products, carbon nano-materials do not sufficiently exhibit their characteristics.
In the past, as means of improving the dispersibility of carbon nano-materials, for example, a carbon nano-material having a surface oxidized by adding an oxidant to an acidic suspension of a fine carbon fiber (PTL 1), carbon nano-material into which a COOM group is introduced through wet oxidization using nitric acid or an mixed acid of nitric acid and sulfuric acid (PTL 2), a carbon nano-material into which a nitro group is introduced through an ultrasonic treatment in fuming nitric acid or an mixed acid of fuming nitric acid and concentrated sulfuric acid (PTL 3), and the like are known.
However, in the above-described examples of the related art, there was a problem in that the conductivity deteriorated due to the carbon nanofiber cut by an excessive oxidization treatment or the dispersibility was not sufficient due to an insufficient oxidization treatment, or there as a problem in that, since an organic solvent was used as a dispersion medium during nitration, the dispersibility of the carbon nanofiber was not sufficient at a high concentration.